Moscow: Leading critics of President Vladimir Putin on Tuesday urged Britain to step up pressure on the Kremlin chief by imposing sanctions on visiting Russian government officials and oligarchs.

Former prime minister Mikhail Kasyanov, members of the punk rock band Pussy Riot and environmental campaigner Evgenia Chirikova also urged Western leaders to stay united against Russia over Ukraine.

"Britain can help the whole world," Chirikova said at a conference in a parliament building in London on the fifth anniversary of the death in a Moscow prison of Sergey Magnitsky, a lawyer who made repeated accusations against Putin and the Kremlin.

"Why do Russian oligarchs feel so at ease in Britain? Do you like the war in Ukraine?" she told the audience, which included dozens of lawmakers.

"You can make these people`s lives miserable and they in turn will stop Putin," said Chirikova, who came to prominence with a campaign against the building of a new road through a forest near Moscow.

Nadezhda Tolokonnikova from Pussy Riot, who spent 22 months in a Russian prison along with band member Maria Alyokhina, congratulated Western leaders for standing up to Putin at the G20 summit last weekend in Australia.

"We were very surprised to see that European politicians started to behave like punks... We felt that our work is not just empty words," said Tolokonnikova, who now runs a prisoners` rights charity with Alyokhina.

Kasyanov said Putin too had been "surprised" by the unity among European nations and the United States against Russian actions in Ukraine in recent months.

"It`s important to keep a principled position," he said.

Europe and the United States have ramped up asset freezes and visa bans on officials accused of wrongdoing and economic sanctions against Russia as the conflict in southeast Ukraine has spiralled.

British Prime Minister David Cameron said the G20 summit sent a "very clear message" to Russia to stop meddling in Ukraine or face more sanctions.

The West accuses Russia of aiding separatist rebels in Ukraine with troops and heavy armour, a claim denied by Moscow which accuses the government that seized power in February of being undemocratic.